The electrostatic coating process for the application of color coatings on various substrates is well known. The composition which is utilized in this process is an electrically chargeable powder mixture of a film-forming polymer and a pigment (or pigments) having the desired color. Unfortunately, preparation of such powder coating compositions for application by a powder spray applicator has presented a problem in the past in connection with platy pigments.
Platy pigments of a metallic material rely on their laminar structure for maximum appearance effect. Such plates include, for instance, metallic flakes such as aluminum, bronze and stainless steel plates as well as natural or synthetic pearlescent pigments exemplified by, e.g., natural pearlescence or a metal oxide- coated substrate such as titanium dioxide-coated mica, iron oxide-coated mica, titanium dioxide-coated glass, iron oxide-coated glass and iron-coated aluminum flakes. The laminar structure of such metallic or pearlescent pigments is destroyed during the extrusion or grinding processes which are used in the manufacture of powder coating compositions and as a result, the coating appearance achieved exhibits a reduced luster effect.
The industry has attempted to avoid the foregoing problem by dry blending the pigments and polymer powder, i.e., the polymer carrier is mechanically blended with the pigment. Unfortunately, the pigment and the powder particles usually develop a different charge magnitude which results in a color shift when the powder coating composition is electrostatically sprayed on a substrate. In addition, the pigments tend to separate from the powder in that volume of the powder spray composition which does not attach to the substrate and is then recovered, which makes that recovered material difficult to reuse.
To overcome the problems encountered as a result of the dry blending process, a blending process in which the powder base and the pigment were mixed and then heated to a temperature sufficient to soften the surface of the powder particles so that the pigment could bind to the surface of such particles was developed, as described in U.S. Pat. 5,187,220. While this process works well for corona electrostatic charging and alleviates the problems with reusing the powder overspray, it also leaves two distinctly different surfaces in the final mixture. The procedure also does not work well for tribo electrostatic charging.
A significant advance in the art is described in U.S. Pat. No. 5,824,144, the disclosure of which is hereby incorporated by reference. As there described, the metal containing platelet pigment is provided with a viscous surface layer of polymer or other sticky liquid material. When that treated pigment is blended with powder coating composition, the powder attaches to the surface of the pigment thereby minimizing color separation. The pigment is also encapsulated by the powder to thereby present a single surface. A better charge of those pigment particles which do not become attached to the powder materials is also realized.
A significant problem in connection with the use of metal-containing platelet pigments in a powder coating composition is that the concentration of the pigment which can be incorporated is limited. While pigment amounts of up to about 10% or more can be utilized in single application environments or in the laboratory, the concentration is limited to about 3% when operating on a larger scale as is required commercially. At levels higher than about 3%, numerous problems arise. These include spray gun clogging, blotchy panel appearance and color separation.
It has now been discovered that a treatment which had previously been developed for pearlescent pigments intended for exterior use to provide improved humidity, resistance and overall weather ability and which was used in liquid coating systems such as a solvent or water borne automotive paint systems, for example, pigmented base coat and clear top coat, provides superior properties in the context of powder coatings. The exterior pigment treatment surprisingly improves the application properties of the pigment when incorporated into powder coatings. Such improvements include less build up of the pigment at the electrostatic gun tip, improved transfer through feed lines, improved transfer efficiency of the pigment and a more uniform coating appearance.